'an unknown error occurred and the upload could not be saved' in product images

Hi,
I am trying to upload product images to to my shoppe and I get a pop up saying 'an unknown error occurred and the upload could not be saved'

I have just upgraded to the 3.7.1 and I think it's only just happened since. It's always been fine. The images upload to the Media Library but I cant work out a way to add them from there to my product images.

Please help - I need to get these images into the shop.

I'm can only do the 'normal' stuff - I'm not into coding and stuff - so please be simple with me.